Sounding reference signals (SRS) are one type of reference signals that are transmitted by mobile terminals so that the base station can estimate the uplink channels used by the mobile terminal. In the context of Long Term Evolution (LTE) the mobile terminals are referred to as User Equipments (UEs) and the base stations are referred to eNBs. The channel estimates may be used for uplink scheduling and link adaptation but also for downlink multiple antenna transmission, especially in case of TDD (Time Division Duplex) in LTE where the uplink and downlink use the same frequencies. The sounding reference signals are in part defined in technical specification 3GPP TS 36.211 and have time duration of a single OFDM (Orthogonal Frequency Division Multiplex) symbol. In the uplink of LTE system a special type of OFDM, called DFTS-OFDM (Discrete Fourier Transform Spread OFDM), is used.
Sounding reference signals can be transmitted in the last symbol of a 1 ms uplink subframe. This is illustrated in FIG. 1 where a frame of 10 ms is shown. Each frame is divided into 10 subframes and one subframe can carry 12 or 14 symbols depending on whether a normal cyclic prefix or extended cyclic prefix is used. In LTE, the uplink symbols are DFTS-OFDM (Discrete Fourier Transform Spread-Orthogonal Frequency Division Modulation) symbols which also are referred to as SC-FDMA (Single Carrier-Frequency Division Multiple Access).
For the case with TDD, the sounding reference signals can also be transmitted in the special time slot called UpPTS, which is the uplink part of a special subframe. The special subframe further comprises a downlink part, DwPTS, and a guard period. The length of UpPTS can be configured to be one or two symbols. FIG. 2 shows an example for TDD with 3 downlink subframes, 2 uplink subframes within a 10 ms radio frame. Up to eight symbols may be set aside for the sounding reference signals.
The configuration of SRS symbols, such as SRS bandwidth, SRS frequency domain position, SRS hopping pattern and SRS subframe configuration are set semi-statically as a part of RRC information element defined in the technical specification 3GPP TS 36.331. There are in fact two different configurations                Cell specific SRS configuration, which is further described in 3GPP TS 36.211.        UE specific configuration, which is further described in 3GPP TS 36.213.        
The cell specific configuration indicates to all UEs in a cell in which subframe the last symbol of the subframe, or the special slot referred to as UpPTS, can be used for sounding.
In normal uplink subframes, PUSCH (Physical Uplink Shared Channel) transmission which overlaps in the frequency domain with a cell specific subframe will be shortened in the sense that the last uplink symbol is not used for data transmission in the uplink. Furthermore, in subframes where cell specific SRS resources have been reserved, terminals in the cell should use shortened PUCCH (physical Uplink Control Channel) format when transmitting ACK/NACK or scheduling requests, which is referred to as formats 1a/1b/1. The shortened PUCCH format implies that the last symbol in the subframe is not used for transmission of control information. There are also certain rules that should be applied when the UE shall transmit the SRSs. For example, the UE shall not transmit an SRS in a subframe where a CQI (channel quality indication) report using PUCCH format 2 is transmitted.
The UE specific configuration is as the name indicates UE specific and indicates to the terminal a pattern of subframes and frequency domain resources to be used for the SRS. The UE specific configuration also indicates other parameters that the UE shall use when transmitting the SRS, such as frequency domain comb and cyclic shift.
This means that SRSs from different terminals can be multiplexed in the time domain, by transmitting the SRS in different subframes. Furthermore, within the same symbol, sounding reference signals can be multiplexed in the frequency domain. The set of subcarriers is divided into two sets of subcarriers, or combs with the even and odd subcarriers respectively in each such set. Additionally, the UEs may have different bandwidth to get additional Frequency Division Multiplexing (FDM). The comb enables frequency domain multiplexing of signals with different bandwidths and also overlapping. Additionally code division multiplexing (CDM) can be used. Then different users can use exactly the same time and frequency domain resources by using different shifts of a basic base sequence. An example illustrating the multiplexing possibilities is given in FIG. 3 where it is shown how SRS from different users can be multiplexed in the frequency domain and the time domain.
In LTE Rel-8/Rel-9, resources for transmission of sounding reference signals are reserved on a cell level by higher layers. Users in a cell are then semi-statically configured to transmit sounding reference signals using the resources reserved in the cell. When resources are reserved for sounding, they are taken from the cell and hence from all user in the cell. As a result regardless of whether there is a need for the UEs to sound the channel or not, the resources are taken from all users and can not be used for data transmission.
In a cell with a certain number of users, the period of SRS transmission is specified by the number of users and the bandwidth of SRS. When a large number of users share the SRS resources, the SRS period might become too large for some users, such as users with fast changing channels who need more frequent update from their channels, while for some other users this period may be too frequent.